Garage Doors: Usually 2.1m wide. Thankfully the rib manufacturers have realised this, and a lot of the 4/5m hulls are 2m beam. Not a lot of error margin, but they'll fit!
As for trailers, unbraked you shouldn't have a problem, as most are small due to the fact they can't take a lot of weight (max unbraked tow is 750Kg, and ony then if your car weighs 1.5 tons minimum.) For a 4m you should be OK unbraked unless you are hitching it to the back of a Vauxhall Nova or similar. Width wise Unbraked I would be amazed if it didn't fit through, braked axles are a whole different kettle of fish - I only found 2 that would go through the door......
Here is the winch post thread:
Setting up trailer for SR4
I should add that I have since added a 3" spacer between the winch & the post so the pull for mthe winch is now forward. The "clever" bit is getting the winch about halfway down the braked hitch - saved me about half a metre.